A device introducing unburned fuel vapor into an intake pipe of an engine is adopted in the related art to improve fuel consumption of the engine. For example, in a device described in Patent Literature 1, fuel vapor generated in a fuel tank is trapped temporarily in a canister. The fuel vapor trapped in the canister is forced out from the canister and introduced into the intake pipe by a negative pressure which develops in the intake pipe when intake air in the engine flows the intake pipe. The device described in Patent Literature 1 detects an internal pressure of the fuel tank and also detects an abnormality in an introduction pathway of fuel vapor including the canister according to a detection value of the internal pressure.